Electrophotographic marking is a well known and commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a light image representation of a desired document onto a substantially uniformly charged photoreceptor. In response to that light image the photoreceptor discharges so as to create an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image so as to form a toner image. That toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
The foregoing broadly describes a black and white electrophotographic printing machine. Electrophotographic marking can also produce color images by repeating the above process once for each color of toner that is used to make the composite color image. For example, in one color process, referred to herein as the REaD IOI process (Recharge, Expose, and Develop, Image On Image), a charged photoreceptive surface is exposed to a light image which represents a first color, say black. The resulting electrostatic latent image is then developed with black toner particles to produce a black toner image. A recharge, expose, and develop process is repeated for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say cyan. The various color toner particles are placed in superimposed registration so that a desired composite color image results. That composite color image is then transferred and fused onto a substrate.
The REaD IOI process can be implemented in various ways. For example, in a single pass printer wherein the composite final image is produced in a single pass of the photoreceptor through the machine. A second implementation is in a four pass printer, wherein only one color toner image is produced during each pass of the photoreceptor through the machine and wherein the composite color image is transferred and fused during the fourth pass. REaD IOI can also be implemented in a five cycle printer, wherein only one color toner image is produced during each pass of the photoreceptor through the machine, but wherein the composite color image is transferred and fused during a fifth pass. Furthermore, in addition to the REaD process there are also other schemes for electrophotographically producing a color image. However, multiple developers can be expected.
In electrophotographic printing the developer actually conveys toner onto a latent image. In development, charged toner particles are applied to a latent image such that toner particles electrostatically adhere to the proper areas of the latent image. There are many types of developers. One is the so called magnetic brush developer that uses a two-component developer comprised of toner particles and magnetic carrier beads. The toner particles triboelectrically adhere to the relatively large magnetic carrier beads. When the developer material is placed in a magnetic field, the carrier beads and their toner particles form relatively long chains which resemble the fibers of a brush, thus the name. Electrostatic charges on the photoreceptor cause the toner particles to be pulled off the carrier beads and onto the photoreceptor. Another developer is the "scavengeless" developer, see U.S. Pat. No. 4,868,600 to Hays et al., U.S. Pat. No. 4,984,019 to Folkins, U.S. Pat. No. 5,010,367 to Hays, and U.S. Pat. No. 5,063,875 to Folkins et al., and their citations. In scavengeless development toner is conveyed onto a latent image using AC electric fields that are applied to electrode structures, commonly wires, that are positioned between a toner loaded donor roll and the photoreceptor. A variation of the scavengeless developer is the "hybrid" scavengeless developer, or HSD. A HSD includes a developer housing with a toner reservoir, a transport roll, a donor roll, and an electrode structure. The transport roll operates like a developer roll, but instead of conveying toner particles directly onto the photoreceptor the transport roll conveys toner particles onto a donor roll that is disposed between the transport roll and the photoreceptor. The transport roll is electrically biased such that toner particles are attracted from the transport roll. When loaded with toner the donor roll can then convey toner particles toward the photoreceptor. To do so the electrode structure is AC-biased relative to the donor roll. That AC bias detaches toner from the donor roll into a toner powder cloud that forms in the gap between the donor roll and the photoreceptor. The latent image on the photoreceptor then attracts toner particles from the powder cloud, developing the latent image. It should be noted that when the donor roll bias and AC bias are removed the toner on the donor roll tends to leave the donor roll and move toward the transport roll and the toner reservoir.
No matter what type of color electrophotography or what type of developer, all of the developers require one or more electrical biases to operate. This requires some type of electrical power supply that must be provided for the developers. In the prior art it was common to provide a separate power supply for each developer. However, because of cost, reliability, and size constraints efforts were expended to develop systems in which only one power supply powered all of the developers. At least one manufacturer may have been successful. Konica's model 9028 is believed to use one power supply that is switched between developers as required.
It should be understood that by saying that only one power supply is used that does not mean that only one voltage is applied to the developers. Rather, that one power supply supplies electrical power required by the developers. For example, a power supply might supply AC for electrode wires, a DC bias for a donor roll, and both AC and a DC bias for a transfer roll. Similarly in some cases we might mean that only one power supply is used to supply only a portion of the biases required, e.g. that a single power supply provides the electrode wire AC but that the DC biases are separately supplied.
While using one power supply may reduce costs and improve reliability, in the prior art only partial success was achieved in reducing the overall system size. When the printing machine sits on a desk size reduction can be very important. One reason that only partial success was achieved has to do with the "standard developer order" (the first developer used is the first developer that a latent image passes, the second developer is the second, and so on). That order restricts the achievable size reduction.
To understand why the standard developer order restricts the achievable size reduction refer to FIG. 1. FIG. 1 shows four developers, the developers 28, 30, 32, and 34 around a photoreceptor 10 that rotates in the direction 12. The developers are powered by a single power supply 76 that is applied via a switch 78 to the individual developers as required. After a first latent image is formed at an exposure station 24 using a laser beam 26, that latent image is developed by the black toner developer 28. Then, a second latent image is formed and that second latent image is developed using the yellow developer 30. Then a third latent image is formed and developed by a magenta developer 32. Finally, a fourth latent image is formed and developed by a cyan developer 34. Note the order of developers, the physically first developer is used first, the physically second is used second, and so on. Also note the distance 80, which is three times the separation between adjacent developers.
After some contemplation it can be seen that, when using only one switched power supply and with the physical order shown in FIG. 1, the separation between the end of one latent image and the beginning of the next latent image, referred to herein as the interdocument zone, must be at least the distance 80. Basically, with only one switched power supply that can power only one developer the end of one latent image must be completely developed (by the developer 34) before the beginning of the next latent image is developed (by the developer 28). This is because generally a single power supply can not simultaneously provide power for more than one developer housing at a time since often either the setpoints of each developer housing are different from one another or the power supply does not have the capacity for multiple loads. In practice the interdocument zone must be somewhat larger since some time is required to switch the power supply between the developers and some more time is required for the developers to become enabled following switching.
Since using one switched power supply to power all of the developers of a color electrophotographic printing system is beneficial, since a small physical size is also beneficial, and since the interdocument zone limits the minimum physical size of a color electrophotographic printing system that uses one switched developer power supply, a technique of reducing the interdocument zone would be beneficial.